The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the compensation for residual magnetization produced by magnetic field gradients in MRI systems.
When a substance such as human tissue is subjected to a uniform magnetic field (static field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned magnetic moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
The application of magnetic resonance to imaging, and to many of the techniques of localized spectroscopy, depends upon the use of linear magnetic field gradients to selectively excite particular regions and to encode spatial information within the NMR signal. During the NMR experiments, magnetic field gradient waveforms with particularly chosen temporal variations are used. Any departure from the application of ideal magnetic field gradient waveforms can, therefore, be expected to introduce image distortion, intensity loss, ghosting, and other artifacts. For example, imperfect rephasing of the nuclear spins and an attendant loss of signal occurs if the slice-select magnetic field gradients are not balanced before and after the 180.degree. RF pulses. This effect compounds in later spin echoes of multi-echo (Carr-Purcell-Mieboom-Gill) sequences. In addition, if the gradient field is not zero when it should be (due to residual magnetization after termination of a gradient pulse), the unintended phase dispersion can result in distorted spectra in chemical shift imaging (CSI) sequences as well as inaccurate spin-spin relaxation time (T.sub.2) determination in multi-echo sequences. Those skilled in the art are thus concerned particularly about the accuracy with which magnetic field gradients are produced.
One source of distortion in the production of magnetic field gradients can arise if the gradient fields couple to conductive structures within the polarizing magnet such as its cryostat (if the magnet is of the superconductive design), or the shim coil system, or the RF shield used to decouple the gradient coils from the RF coil. The induction of currents in these ambient structures is known as eddy currents. Due to eddy currents, one observes, typically an exponential rise and decay of the magnetic field gradient during and after, respectively, the application of a trapezoid current pulse to the gradient coil.
In U.S. Pat. No. 4,698,591 entitled "A Method for Magnetic Field Gradient Eddy Current Compensation," a method is disclosed which uses an analog pre-emphasis filter in the gradient power supply to shape the current applied to the gradient coil in such a way that the eddy current induced gradient field distortions are reduced. The filter includes a number of exponential decay components and adjustable potentiometers which must be set during system calibration. A measurement technique is used prior to system calibration in which the impulse response of the uncorrected magnetic field gradient is measured and the potentiometer settings for the pre-emphasis filter are then calculated. Such techniques are described in U.S. Pat. Nos. 4,950,994; 4,698,591 and 4,591,789.
In iron-core permanent magnets or iron-core enhanced superconducting magnets, there exists another type of gradient-induced magnetic field perturbation. This perturbation, known as hysteresis, has not been well-studied, and generalized correction techniques have not been fully developed. To understand the hysteresis phenomenon, consider the effects of a bipolar gradient waveform shown in FIG. 2 and assume that the iron magnetization is in an initial state 8 shown in FIG. 3. The initial magnetization state is defined as the un-magnetized state, but in this case, it could be the state after the magnetic field is ramped up but before any gradients have ever been applied. During the first attack ramp, the current in the gradient coil, as well as the magnetic field H experienced by the iron core, is gradually increasing. As a result, the magnetic induction B increases with H, as indicated by curve 11 in FIG. 3. When the gradient is ramped down to zero at 12, however, the magnetic induction B does not return to zero. Instead, its dependence on the magnetic field is characterized by another curve 14. This phenomenon is know as hysteresis, and the remaining magnetic induction (.DELTA.B) is called remanence, or residual magnetization. If the gradient is further ramped down at 16 to a negative value, then the magnetic induction B follows curve 18. With subsequent gradient ramp 20, the H vs. B curve 22 ends with a negative residual magnetization (-.DELTA.B). Subsequent gradient pulses drive the magnetization in a loop, known as the hysteresis loop.
The above analysis indicates that when a time-dependent magnetic field gradient pulse is used for imaging, a perturbation magnetic field .DELTA.B can be generated in ferromagnetic materials. If the hysteresis effects are uncompensated, a number of image artifacts can be produced. For example, the residual magnetization induced by the phase-encoding gradient pulses in fast spin echo (FSE) can generate inconsistent phase errors in k-space data, leading to image blurring and ghosting.
This problem is addressed in U.S. Pat. No. 5,729,139. The proposed solution in this prior art patent is to correct the phase errors produced by residual magnetization. Ten specific methods for doing this are proposed and all require changes to the gradient pulse waveforms in the particular prescribed pulse sequence. Since there are countless variations possible in the pulse sequences that can be prescribed, it is not practical to alter each one according to the teaching of this prior method.